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Old 02-12-2005, 10:10 PM
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Pushrod or OHC

Just wondering which one you prefered, discuss the pros and cons of each.
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Old 02-12-2005, 10:18 PM
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i think the pro of pushrod is simplicity of design and it weighs less
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Old 02-13-2005, 12:36 AM
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I rather DOHC for its better breathing and ability to rev
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Old 02-13-2005, 01:37 AM
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Personally I think Push-rods sound better, but if you want something more modern and advanced, go for the DOHC.
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Old 02-13-2005, 01:53 AM
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Quote:
Originally Posted by fpv_gtho
I rather DOHC for its better breathing and ability to rev
This has been a point of discussion all the time when the corvette C6 was being set off against european cars. Protagonists of the C6 invariably claim that pushrods are the best solution, only when you needed revs (and smaller engines) SOHC and DOHC would be a plausible solution. Of course in the US you don't need revs, as there is no replacement for cubic inches. European car tax systems have in some countries been based on displacement (a classic example of the consequences thereof is the existence of the Ferrari 208 turbo, which was the small displacement version of the 308 destined for the Italian market), resulting in optimising smaller sized engines.

So if you don't need revs you would go for pushrods. Fine, but there have been some developments in engine construction have largely escaped the avarage american (and australian) car nut. During the last ten years diesel engine developments have taken off like a space shuttle, and see what happens, these low revving engines (maximum 5000 but never necessary to go over 3500 or maximum 4000) are all fitted with one or two OHC's and 4 valves are now common as well. Why should that be? Is it because it always allows for a much better combustion process because you can place the valves in almost any position you wish? Coould well be, but for me it puts an end to the myth that pushrods are the way to go if you don't need revs.
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Old 02-13-2005, 01:56 AM
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Well the main thing i like about DOHC is its ability to have 4 valves rather than 2, afterall you'll cover more area with 4 valves rather than 2 so you should be able to get better breathing out of the engine from that
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Old 02-13-2005, 02:09 AM
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Quote:
Originally Posted by fpv_gtho
Well the main thing i like about DOHC is its ability to have 4 valves rather than 2, afterall you'll cover more area with 4 valves rather than 2 so you should be able to get better breathing out of the engine from that
you are now not talking any more about abaility to rev?
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Old 02-13-2005, 02:18 AM
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Well....ability to rev is more of a secondary preferance to me. I dont NEED a car to do 7000-8000rpm, where DOHC would have its main advantages over OHV but i'd appreciate the extra valve area moreso. Having the DOHC there anyway makes the engine easier to extract power from by increasing the revs if the situation calls for it
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Old 02-13-2005, 06:12 AM
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OHC avoids the pushrod which can flex.
because OHC acts directly it is better suited for HIGH lift and high spring rates for valve closing and preventing spring bounce.
OHC has no friction from chanig angle as in 'rods, especially if valves are at an angle to the head.
Desmo can ONLY work with OHC thus avoiding springs issues completely.
DOHC enables variation of timing overlap on exhaust and inlet.
OHC doesn't need rockers or pushrods so can have less moving weight than pushrods ( rotating objects in engines are fine, mass going up and down are bad : ) )
Less materials technology required for high performance OHC versus pushrods
DOHC makes the head larger than a pushrod with 2 vlaves/cyl
OHC may make the head slightly larger - but not always as the valves and rockers are still there for an in-block cam.
Pushrod single cam is simpler to time. ( but so is single OHC )
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Old 02-13-2005, 02:28 PM
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depends on what i want. if i want a complicated but hi reving design, but would probably create a little more power, id take the ohc. if i wanted simplicity, lower revs, and othger things that describe pick up trucks and muscle car-type engines, id instantly take the pushrod
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Old 02-13-2005, 02:32 PM
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Quote:
Originally Posted by CdocZ
depends on what i want. if i want a complicated but hi reving design, but would probably create a little more power, id take the ohc. if i wanted simplicity, lower revs, and othger things that describe pick up trucks and muscle car-type engines, id instantly take the pushrod
what you described as your second option is best fulfilled with a diesel engine and yet all modern diesel engines run one or two OHC's. Got an explantion for that?
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Old 02-13-2005, 02:35 PM
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kind of. not too good with words. ill try to rephrase
depends on what you want, hi revving hi tech engine, or low rev low tech but most likely more reliable.
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Old 02-13-2005, 02:39 PM
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See, the "complexity" one gets repeated so often adn yet isnpt true !!

The crankshaft has a gear/chain/belt drive to the camshaft in BOTH.
Each lobe on the cam drives a bucket, pushrod to rocker to valve stem.
OR each lobe on the cam drives a bucket to the valve stem.

So often folks talk about complication because there are MORE cams with DOHC.
But they're there for a reason.

THAT is a choice the engine designer makes. If he/she just wants one cam then it's dead easy - the Ford Pinto's been doing it for 20 years in competition. In SOHC , multi-valve you usually have to add rockers to prevent twist..

So IN FACT all things being equal - valves/cylinder - there are MORE parts with pushrods.

The ONE plus in a V8 configuration is the designer can get away with only one camshaft, whereas with a OHC there MUST be one per bank. However, that is counter-acted that if an 'event ' causes damage and the cam needs replaceing ( usually along with bent valves, rockers and followers ) it's a LOT easier on overhead as only one bank needs stripped.
Where does complexity come into it ?
I reckon it's because so few folks have actually STRIPPED and rebuilt and engine. So from pics OHC looks "complicated" because you can SEE most of the parts that are hidden deap in the engine with 'rods.

Maybe one of the folks who think it IS complicated coudl explain to me, please, I do need to understand because it gets used ALL the time as a drawback that isn't matched by the reality.
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Old 02-13-2005, 02:40 PM
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Quote:
Originally Posted by CdocZ
kind of. not too good with words. ill try to rephrase
depends on what you want, hi revving hi tech engine, or low rev low tech but most likely more reliable.
it it not a matter of how you described things. Diesel engines provide great amounts of low end torque and will very rarely have to go over 4000 revs. In fact they show all the charactistics of a traditional "lazy" petrol engine, and yet, all modern ones are fitted with OHC's. So both modern high revving and low revving engines produced in Europe (and Japan) are fitted with OHC's. In my opinion this means that for choice options you describe OHC's provide the best option.
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Old 02-13-2005, 02:42 PM
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yeah.........i really need to do some more reading on deisel cars
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